Activated carbon having an excellent adsorption property is widely used in air cleaning, solvent recovery, flue-gas desulfurization and denitrification, many types of industrial wastewater treatment, and other applications, in addition to its use in catalyst and catalyst carrier applications. In recent years, electric double-layer capacitors are garnering attention as backup power sources and auxiliary power sources. Development is being widely conducted with attention given to the performance of activated carbon as the electrodes of an electric double-layer capacitor. Electric double-layer capacitors that use activated carbon as polarizable electrodes have excellent electrostatic capacity, so demand is growing rapidly in concert with the development of the electronics field. In addition to the miniaturization of conventional power sources for memory backup, and the like, in recent times, development of large capacity products such as those used in auxiliary power sources for motors, and the like, is being conducted.
The principles of electric double-layer capacitors have been long known, but these capacitors have only started actually being used as devices in recent years. The electrostatic capacity of electric double-layer capacitors mainly depends on the surface area of the polarizable electrodes that form the electric double layer, the resistance of the electrodes, the capacity of the electric double layer capacitor per unit surface area, and other factors. In actual application, it is important to increase the density of the electrodes themselves in order to increase the electrostatic capacity per unit volume and to decrease the volume of the electric double-layer capacitor. Conventionally, activated carbon applications for electric double-layer capacitors have included (1) activated carbon obtained by activating resin material, coconut shells, pitch, coal, and the like under acidic conditions using steam, gases or the like (See “Daiyoryo Kyapashita Gijutsu to Zairyo” (Technology and Materials for EDLC), CMC Publishing Co., 1998), (2) activated carbon obtained by a method in which materials applied in (1) described above have been activated by KOH and other highly oxidative chemicals (Japanese Patent Application Laid-open No. 10-199767, and other publications), and the like.
As described above, high electrostatic capacity and low resistance are required for electrodes used in capacitors. However, when activated carbon obtained with the method described in (1) above is used, sufficient electrostatic capacity cannot be obtained, and a bulky device is needed in order to obtain the required electrostatic capacity. According to the method disclosed in (2) described above, a somewhat high-capacity activated carbon can be obtained by activation with a highly oxidative activator such as potassium hydroxide. However, conventional alkali activation is an approach in which an alkali is heated together with a carbonaceous material to create a slurry, as disclosed in Japanese Patent Publication No. 62-61529, and handling an alkali in a dangerous melted state is inevitable. In addition to such safety problems, conventional alkali-activation technology for carbonaceous material does not take corrosion of the device into consideration at all. Therefore, it is difficult to say that this technology can be advantageously implemented on an industrial basis.
An activated carbon made from a granular isotropic pitch serving as a raw material is disclosed in Japanese Patent Application Laid-open No. 2002-104817, which describes a method for manufacturing activated carbon comprising a wetting step for pitch that wets at least the surface of a granular mixture of KOH and isotropic pitch at 200° C. or less, a solidifying step for pitch that eliminates the wetting step at 400° C. or less to create a solid state, and a heat treatment step for pitch that performs heat treatment at a temperature greater than 400° C. while maintaining the solid state. To reduce the alkali-induced corrosion of a device, the contact between the alkali and the device needs merely to be reduced as much as possible, and from this viewpoint it can be said that treatment in a solid state is a very effective method. However, according to the disclosure in the above-stated publication, it is difficult to maintain a shape and it is apparent that this method still does not solve the problem of alkali-induced corrosion in a device.
An object of the present invention is to provide a method for manufacturing activated carbon that can reduce corrosion of a device by alkali, and that has excellent stability; to provide a polarizable electrode in which this activated carbon is used; and to provide an electric double-layer capacitor in which this polarizable electrode is used.